Fluorescence occurs when a material emits visible light after being excited by an excitation source applied from outside. A fluorescent lamp, a discharge tube, and a cathode ray tube utilize fluorescence. A material that emits fluorescence is called a phosphor.
Electroluminescence is a solid state phenomenon, which involves the emission of visible or invisible radiation as a result of the absorption of exciting energy. It is a general term which includes both fluorescence and phosphorescence. Invisible light further includes infrared and ultraviolet radiation.
An electroluminescent (EL) display device generally includes a layer of phosphor positioned between two electrodes, with at least one of the electrodes being light-transmissive. At least one dielectric also is positioned between the electrodes so the EL display device functions as a capacitor. When a voltage is applied across the electrodes, the phosphor material is activated and emits a light.
Phosphors may be employed in the manufacture of electroluminescent devices. Long-lasting phosphors are known in the art, and include sulfides and oxides. Many long-lasting phosphor products are those with a sulfide as their base crystal, such as ZnS:Cu. These are disadvantageous in that the after-glow lasts for a relatively short duration of time, for example, for about three hours at the longest.
Phosphorescence characteristics are influenced by composition, particle diameter, and environment, in particular, the phosphorescence brightness of phosphors. With respect to particle diameter, there is a tendency that the phosphorescence brightness decreases proportionally with a decrease in the particle diameter below 100 microns.
Light-emitting small molecules (LEMs) may also be employed in the manufacture of electroluminescent devices. Suitable light-emitting small molecules include quinolines, fluorescein, and the like.
Light-emitting polymers (LEPs) may further be employed in the manufacture of electroluminescent devices. Suitable light-emitting polymers include MEH-PPV (2-methoxy-5-2′-ethylhexyloxy)-1,4-phenylenevinylene copolymer, MEH-BP-PPV (poly[2-Methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene-co-4,4′-bisphenylenevinylene]), and MEH-CN-PPV. These LEPs absorb radiation at about 400 to about 500 nm (blue light) and emit radiation at about 600 and 800 nm (yellow, orange, and red light).
The short lifetime of organic light-emitting polymers (LEPs) is presently a major impediment to their use in commercial environments. Organic LEPs are unstable when exposed to air and humidity. In addition to oxygen, other contaminants present in air, such as ozone and NH3, also adversely affect the useful lifetime of LEPs.
Heretofore, lamps fabricated from LEPs have been entirely encapsulated, or have had exposed surfaces coated with protective layers to achieve stability. This large-scale encapsulation/coating process is costly, and requires the use of a relatively expensive transparent material.
In addition, the phosphors used in previous EL devices require relatively high voltage, typically in the range of about 60 to about 240 volts. What is needed is an electroluminescent device that requires minimal operating voltage and that exhibits long term stability without stringent inert atmosphere handling or encapsulation requirements.
Thus, it would be a contribution to the art to provide phosphor particles coated with a suitable light-emitting substance, wherein said substance may be molecular or polymeric in nature, or a combination of both, having enhanced stability for use in electroluminescent devices, as well as electroluminesecent devices employing same. It would be a further contribution to the art to provide phosphor particles layered in an electroluminescent device with additional layers of other suitable light-emitting substances, wherein said substances may be molecular or polymeric in nature, or a combination of both.